Recently, for steam turbine plants, increasing the temperature of steam has been discussed to improve the thermal efficiencies of plants.
Conventional steam turbine plants generally introduce a one-stage reheating configuration using reheated steam. In the steam turbine plant with the one-stage reheating configuration, steam at a temperature of about 1000 degrees Fahrenheit is used for a high pressure turbine, while steam at a temperature of 1000 or 1050 degrees Fahrenheit is used for an intermediate pressure turbine as reheated steam.
According to the Rankine cycle, which is a thermal cycle generally used in a steam turbine plant, when the steam temperature is increased, the plant thermal efficiency can be more improved.
A conventional high pressure turbine and intermediate pressure turbine for a steam turbine plant is described in Japanese Patent Application (Kokai) No. 11-350911. In this publication, the intermediate pressure turbine uses steam at a temperature about 1100 degrees Fahrenheit as reheated steam, the turbine having a reheated steam supply tube with a steam-cooled double-tubing structure.
However, such a system cannot effectively operate with a temperature of the reheated steam above about 1300 degrees Fahrenheit (or about 700 degrees Celsius), and there remain problems to be solved.
To supply such high temperature steam in a steam turbine, one area of investigation is adopting a nozzle box, including in an intermediate pressure turbine.
A nozzle box is a chamber for steam that is supplied from a boiler or the like, and supplies the steam to turbine stages in the turbine. A nozzle box preferably also functions to avoid constituent components surrounding the nozzle box from direct exposure to high temperature steam.
Conventionally, a high pressure steam turbine has a nozzle box positioned at an inlet side of the steam; however, it is designed to maintain its strength when the steam is at a temperature of 1050 to 1100 degrees Fahrenheit. it is difficult for a conventional nozzle box to maintain its strength with steam at a temperature of 1300 degrees Fahrenheit or above. Further, for such high temperature steam, a radiation problem is to be considered. When adopting steam at about 1300 degrees Fahrenheit, a heat transfer by radiation is not negligible because the temperature of outer surfaces of the nozzle box becomes high. The radiation heat will undesirably heat up constituent components, such as a casing and a turbine rotor, that are provided around the nozzle box.